The present invention relates to a filtering apparatus and a method for filtration of a suspension.
Filtering apparatus of this kind are used, among other things, in the purification of scrubber water, industrial sewage, drinking water and also in the purification of chemicals, wherewith liquid-carried contaminants and solids are filtered out as the liquid passed through a bed of filter particles, said filter particles normally being sand although not necessarily so. The filter medium may, for instance, be activated carbon. As the liquid passes through the sand in the filtering process, particles carried by the liquid will adhere to and contaminate or dirty the sand particles. It is therefore necessary to recondition the contaminated sand, such reconditioning conventionally being effected in a filter-medium washer located adjacent the actual filtering plant.
Filtering apparatus of this kind which include a filter-medium washer are known, for instance, from SE 7602999-0, SE 7613367-7 and SE 8801959-1.
With filtering apparatus of this kind, the liquid to be filtered is pressed up through a bed of sand held in a container and the filtrate leaves the container through an outlet located above the bed. The sand is dirtied by the liquid-carried contaminants most heavily at the bottom of the bed. Cleaning of the sand is carried out continuously, by passing sand from the lower part of the bed and up to the filter-medium washer, where the sand passes down through a wash chamber with the aid of gravity. At the same time, washing liquid is pressed up through the wash chamber and washes the sand. The sand falls from the wash chamber and passes to the upper part of the filter bed. Part of the filtrate is used as washing liquid and the resultant dirty washing liquid is then discharged as reject water.
In the case of SE 7602999-0, for instance, the filter-medium wash is located within the container that encloses the sand bed. Dirty sand is shifted to the upper part of the wash chamber with the aid of an airlift pump, while washed sand falls from the lower part of the wash chamber down onto the upper side of the bed.
In order for filtering apparatus of this kind to function optimally, the design of the filter-medium washer must be well tuned with respect to other apparatus components and to the prevailing operating situation. This can be difficult to achieve at times. On the one hand, the filter-medium washer will preferably have the smallest possible vertical extension, so as to avoid unnecessarily high plant. On the other hand, the washer shall be sufficiently large to ensure that the sand will be washed satisfactorily. The throughflow of washing liquid is also a critical parameter. An optimum in this respect constitutes a compromise between the requirement of a large flow so as to obtain the best possible washing effect, and the desire to restrict the flow so as not to unnecessarily waste washing liquid. Since the washing liquid is normally taken from the filtrate generated in the filtering process, it is desirable that the least possible amount is lost in the form of reject liquid.
Known apparatus of this kind are inflexible with respect to achieving effective washing of the soiled filter medium under the aforesaid conditions. In many cases, this results in plant that cannot operate optimally and which, in the worst case, can only be made to function satisfactorily with difficulty.
The object of the present invention is to solve this problem and to provide filtering apparatus and a filtering method which enable the filter-medium washer to be adapted optimally with respect to other plant components and also with respect to the operating situation in question.
According to a first aspect of the invention, this object is achieved with an apparatus of the kind defined in the preamble of Claim 1 and having the significant features set forth in the characterising clause of said Claim.
As a result of the regulating devices with which the apparatus is provided, the washing process can be optimised with respect to the vertical extension of the wash chamber such that said chamber will have an extension that ensures that the filter medium will be washed to the extent required without needing to over-dimension said chamber and make the apparatus large and bulky.
One simple and purposeful method of regulating the vertical extension of the wash chamber is to construct said chamber from a number of modules that can be connected sealingly to one another and disconnected from each other such as to enable said vertical extension to be varied either by adding or removing an appropriate number of modules, this being done, for instance, during the trimming phase of the apparatus. Consequently, an apparatus of this design constitutes a preferred embodiment of the invention.
In the case of a further preferred embodiment of the invention, the washing-liquid outlet means includes a vertically movable spillway. Adjustment of the position of the spillway influences the counterpressure against which the liquid acts as it flows through the wash chamber, and therewith also influences the rate of flow of said liquid.
The filter-medium washer will also preferably be arranged within the vessel included in the filtering apparatus, so as to obtain a compact plant, and so that the flows of filtering medium and liquid through the apparatus can be favourably formulated.
The filter-medium washer will conveniently include fully vertical walls, so that the force of gravity can be utilised in the best way in the process and stagnation of the filter medium in the wash chamber avoided.
A suitably designed wash chamber will include two cylindrical walls that form a ring-shaped wash chamber into which flanges extend from said walls. According to one preferred embodiment, each module is comprised of a wall-part that includes at least one flange.
In another preferred embodiment of the inventive apparatus, the washing liquid is comprised of filtrate arriving from the bed and collected in a container located above the bed. Filtrate is taken from the filtrate container through a clean liquid outlet. The pressure generated in the container is used to press filtrate through the wash chamber, where it is used as washing liquid. According to one preferred embodiment of the invention, the vertical position of the clean-liquid outlet can be adjusted, so as to adjust the pressure in said container and therewith the flow of washing liquid through the wash chamber.
The aforesaid embodiments of the inventive filtering apparatus and other preferred embodiments of said apparatus are set forth in the Claims appended to Claim 1.
According to a second aspect of the invention, the object of the invention has been achieved by means of a method of the kind defined in the preamble of Claim 14 and comprising the method steps set forth in the characterising clause of said Claim.
The inventive method affords advantages similar to those described above with reference to the inventive filtering apparatus. Advantageous embodiments of the inventive method are set forth in the Claims dependent on Claim 14.